In the known structural configurations of nuclear reactors, the pressurized enclosure of the primary loop performs a pressure-containing function and a sealing function, such that the release of fission products to the environment in normal operation and in design base events is limited to values below maximum permissible limits.
In certain types of nuclear reactors, both the pressure-containing function and the sealing function for the primary loop are performed by a single wall steel pressure vessel. The loss of the pressure containment function, such as in a catastrophic failure of the pressure vessel, is prevented by redundant measures, for example, design for basis safety, quality control and operational surveillance. As it is assumed that leakage will precede fracturing, a safety container surrounding the steel pressure vessel is required as a redundant barrier against the release of fission products. In this manner, the pressurized containment of the primary loop is secured against failure even in relation to the sealing function. A safety container as a redundant barrier against the loss of the sealing function is required primarily because the reactor coolant has a high radioactivity content. Nuclear reactors of this type are described, for example, in German Auslegeschrift No. 23 15 318 and German Offenlegungsschrift No. 23 15 319.
In the case of a high temperature reactor with spherical fuel elements, the fissionable material is coated to form the spherical elements, which are then embedded in a graphite matrix. The coating and the graphite matrix form a first barrier and a second barrier, respectively, against release of fission products. In addition, two further barriers are provided against the release of fission products. These are formed by the reactor pressure vessel (with a sealing function and a failure-safe pressure containment function) and the protective reactor housing (with a sealing and a pressure containment function). The reactor pressure vessel may be made of steel or prestressed concrete and the protective reactor housing of steel or concrete.
A nuclear reactor of this structural type is known for example from German Offenlegungsschrift No. 32 12 322 which corresponds to United States Application Ser. No. 718,193, in continuation of Ser. No. 481,749. In such an installation, construction costs are very high, because the protective reactor housing must function not only as a safety container for the primary loop, but must also act as a structural enclosure for the reactor pressure vessel and the operating installations of the reactor, that is, it must function as the protective barrier against effects from the external environment.
In a high temperature reactor with a prestressed concrete pressure vessel, all of the components of the primary loop are located in a cavity (or several cavities) of the prestressed concrete pressure vessel, which, for installation and disassembly and to receive the components, comprises a plurality of large passages. Tight enclosure of the primary loop is effected in the area of these passages by means of closure devices, which close off the passages in a pressurized and gas-tight manner. A nuclear reactor with a prestressed concrete pressure vessel comprising such passages and closure devices is described in German Offenlegungsschrift No. 31 41 734. The closure devices consist in each case of a double steel cover set into the passage involved. The inner cover represents the seal for the primary gas. In this installation, again, a tight protective reactor building must be present.
German Offenlegungsschrift No. 15 14 783 also discloses the principle of using double covers as a safety closure for nuclear reactor pressure vessels.